Cervix positioning to determine conception ?

Well as i have for-mentioned in a previous post, I think ovulation occurred on the 15th or 21st ( Been taking OPKS and only got high results since 15th (CD18) ) I have also been checking my cervix every morning and night after emptying my bladder. I have found that it has been tightly closed during the morning and then when I check again it is slightly opened and viseversa at night time. I have been cramping a lot the past 3 days and it feels as if I am bloated and just needing the loo to empty my bowels but nothing's happening just wind. Could i possibly just getting my period early? It's not due until 29th of this month tho. Any advice would be greatful if you have been in this situation before . Many thanks